1. Field of the Invention
The present invention relates to a switching power supply for supplying power that is provided to a load by turning a switching element on and off according to a control circuit.
2. Description of the Related Art
As a conventional switching power supply, a switching power supply 30 disclosed in Japanese Unexamined Patent Publication No. 5-260732, and in particular an auxiliary power supply circuit thereof, will be described with reference to FIG. 5.
A series circuit including a Zener diode Zd and a resistor R3 is connected to both terminals of a direct current (DC) primary power supply E. A PNP transistor Q6 that constitutes a start switching circuit s1, and a PNP transistor Q7 that constitutes a steady-state-operation switching circuit s2 are connected such that the emitters thereof are commonly connected to a negative terminal of a control circuit s3. Further, a positive terminal of this control circuit s3 is connected to a positive terminal of the primary power supply E.
The base of the transistor Q6 is directly connected to the connection point of the Zener diode Zd and the resistor R3, and the base of the transistor Q7 is also connected thereto through a diode D7. Accordingly, this diode D7 is connected between the bases of the transistors Q6 and Q7. Further, the collector of the transistor Q6 is connected to a negative terminal of the primary power supply E through a resistor R4. Moreover, the collector of the transistor Q7 is connected to a negative terminal of a DC auxiliary power supply V. Furthermore, a resistor R5 is connected between the base and collector of the transistor Q7.
A power conversion circuit p1 includes a switching element, a transformer having primary, secondary and tertiary windings, and a rectifying/smoothing circuit. The DC output power of this power conversion circuit p1 is supplied to a load.
The auxiliary power supply V provides an output voltage by rectifying/smoothing a voltage induced at the tertiary winding of the transformer, set to a voltage lower than the primary power supply E, and supplies power to the control circuit s3 in place of the primary power supply E during steady-state operation. Because the auxiliary power supply V does not generate a voltage when the switching power supply 30 is initialized, power is supplied from the primary power supply E to the control circuit s3 by turning on the transistor Q6, which is used during initialization. After the switching power supply 30 is initialized, the transistor Q7 is turned on, and the transistor Q6 is turned off as the voltage of the auxiliary power supply V connected to the collector of the transistor Q7 becomes equal to or more than a certain value, and stable power is thereby supplied to the control circuit s3 from the auxiliary power supply V through the transistor Q7 in place of the primary power supply E.
This switching-on operation from the transistor Q6 to the transistor Q7 is performed by utilizing the formed voltage drop of approximately 0.7 V across the diode D7 that is connected between the base of the transistor Q6 and the base of the transistor Q7.
As described above, the reason why power is supplied to the control circuit s3 from the primary power supply E during initialization, and power is supplied by switching to the auxiliary power supply V after the initialization is complete is that an operating voltage of the control circuit s3 is lower than the voltage of the primary power supply E. If, without switching, the power were always supplied from the primary power supply E to the control circuit s3, the power loss of the transistor Q6 becomes larger. Accordingly, during steady-state operation, the voltage is lower than the primary power supply B, and by supplying power to the control circuit s3 through the transistor Q7 from the auxiliary power supply V that generates a voltage larger than the voltage required for the control circuit s3, the power loss of the switching power supply 30 is reduced.
However, in an auxiliary power supply circuit (a circuit comprising auxiliary power supply V, the transistors Q6 and Q7 etc.) of the conventional switching power supply 30, since an electrical potential difference of about 0.7 V is applied between the base of the transistor Q6 and the base of the transistor Q7, the transistor Q7 turns on when the transistor Q6 turns on, and thus the voltage applied to the control circuit s3 is about 0.7 V lower than at the time when the transistor Q6 turns off.
Accordingly, as the switching power supply 30 is in a steady-state operation, i.e., in the state where the transistor Q6 turns on and the transistor Q7 turns off, the output current of the switching power supply 30 becomes overloaded and an over-current protection circuit (not shown) operates, and when the output voltage of the switching power supply 30 is lowered, an output voltage of the tertiary winding of the auxiliary power supply V is also lowered, and thus the transistor Q6 turns on again and the power is supplied from the primary power supply E, thereby lowering the voltage of the control circuit s3 by about 0.7 V from that during normal operation.
When the current from the switching power supply 30 is overloaded, a current of the switching element is also larger, and thus it is necessary to supply a sufficient driving voltage to a gate of the switching element. However, in the auxiliary power supply circuit of the conventional switching power supply 30, a voltage applied to the control circuit s3 is lowered by about 0.7 V during over-current protection operation, and thereby there is a problem in that a gate driving voltage of the switching element is also lowered by about 0.7 V, and thus sufficient power is not supplied to the load.
Accordingly, in the auxiliary power supply circuit of the conventional switching power supply 30, in order to prevent a drop in the gate driving voltage of the switching element during over-current protection operation, it is necessary to set the operating voltage of the control circuit s3 during steady-state operation to a value which is higher by that amount. As a result, electronic parts that do not have a high voltage tolerance may not be used in the control circuit s3.
For example, when the control circuit s3 of the switching power supply 30 is constructed by using a low-cost high-speed logic gate IC, since an absolute maximum rated voltage of the high-speed logic gate IC is usually only about 7 V, and by setting the voltage of the control circuit s3 higher to obtain a sufficient gate driving voltage of the switching element, a voltage derating during steady-state operation, in which the voltage rises about 0.7 V, cannot be secured.